Angiogenesis is controlled by the balance between positive and negative regulators. They have identified a novel endothelial cell-specific protein that inhibits the proliferation of endothelial cells but not that of other types of cells examined. The protein, named vascular endothelial cell growth inhibitor (VEGI), also inhibits angiogenesis in cellular and animal models. VEGI is expressed in many normal human tissues. It appears to be a type-II transmembrane protein, and exhibits 20-30 percent sequence homology to members of the tumor necrosis factor family. The gene is markedly upregulated in confluent endothelial cells as compared to proliferating cells. A secreted form of VEGI was overexpressed in CHO cells, and injection of mixtures of the transfected cells with human breast cancer cells in nude mice potently inhibited xenograft tumor growth. They hypothesize that VEGI is a negative regulator of angiogenesis that acts on endothelial cells by an autocrine or juxtacrine mechanism. They will 1) characterize the binding of VEGI to endothelial cells; 2) chemically cross-link the putative receptor to soluble VEGI or to membrane-bound VEGI on an adjacent cell; and 3) determine whether VEGI treatment causes cell growth arrest or apoptosis. Additionally, they hypothesize that upregulation of VEGI in a neovasculature leads to termination of angiogenesis. They will 1) transfect endothelial cells with VEGI under the control of an inducible promoter to determine whether upregulation of VEGI expression causes growth inhibition; 2) transfect breast cancer cells with the inducible VEGI gene to determine whether overexpression of secreted VEGI in xenograft tumors formed by the transfected cancer cells leads to growth inhibition or regression of established tumors 3) determine whether VEGI injected into the circulation of chick chorioallantoic membrane prevents the survival of tumor grafts on the membrane; and 4) determine by in-situ hybridization whether the timing of VEGI gene expression during ovulatory follicle formation coincides with the ending of neovascularization. The studies may not only give rise to valuable insights into the role of this unique gene in the molecular mechanism of angiogenesis regulation, but lead to the development of new antiangiogenic agents of significant clinical value for the treatment of cancer.